Liquid crystal color displays are used in various fields such as displays for computers. With such liquid crystal color displays, it is possible to obtain very highly accurate images.
Many of liquid crystal displays being currently used are transmission type TFT-LCDs. In such TFT-LCDs, light from a back light source is transmitted through liquid crystal cells from the rear of a substrate on which thin film transistors (TFTs) are formed. The polarization state of that light is varied by changing in the orientation of the liquid crystal using the TFTs as switches, and a polarizing plate is used to discriminate between cells which emit specific polarized light and cells which does not emit specific polarized light. However, brightness suffers in this transmission type liquid crystal display because light is filtered through the polarizing plate and color filters. Also, the weight and power dissipation of the back light is a disadvantage in small or portable computers.
For the above reasons, reflection type liquid crystal displays, which have no back light and which ensure a sufficient light quantity using surrounding light appear to be attractive. A schematic diagram of a reflection type liquid crystal display is shown in FIG. 1. This reflection type liquid crystal display is similar to a conventional transmission type liquid crystal display in that it has a liquid crystal interposed between two transparent substrates (which are usually glass substrates). Similarly, a transparent electrode pattern is formed on one of the glass plates formed with a color filter. However, the reflection type liquid crystal display differs from the transmission type liquid crystal display in that the electrode formed on the other glass plate is not a transparent electrode but a metal electrode which serves as a reflecting plate. The light incident on the glass substrate on the color filter side is modulated by the liquid crystal oriented by the voltage applied between the transparent electrode and the metal electrode, and the modulated light is reflected at the surface of the metal electrode and emitted from the glass substrate on the color filter side.
A reflection type liquid crystal display such as this, however, has the disadvantage that sufficient brightness is unobtainable, because of the color filter it employs. About 2/3 of incident white light is absorbed by the color filter. The display does not have high transmittance because of the dyes and pigments are employed as a means of coloring a color filter formed by a normal dyeing method, pigment dispersing method or printing method.
Japanese Published Unexamined Patent Application (PUPA) No. 7-287115, proposes stacking a thin film consisting of a high refractive material and a thin film consisting of a low refractive material to form a multilayered interference film for emitting color without using a color filter. This structure substitutes for a color filter by allowing light of a specific wavelength region to pass through the alternately stacked multilayered film and, by reflecting light of a wavelength region other than that transmitted by the multilayered film. This provides a transmittance of near 100% in the transmitted wavelength region and has a reflectance of near 100% in the reflected wavelength region, with no reduction in the transmittance which is caused by color filters making use of absorption of pigments.
An example of a multilayered interference film is shown in FIG. 2. In that figure multilayered interference films 5, are formed on a glass substrate 1. In FIG. 2, a TiO.sub.2 film 4 is formed on the glass substrate 1, and a SiO.sub.2 film 3 and a TiO.sub.2 film 2 are stacked in sequence on the TiO.sub.2 film 4. Three multilayered interference films such as this are arranged along side each other one for each of the colors red, green, and blue. Here, the refractive indexes of the titan oxide and the silicon oxide are 2.4 and 1.5, respectively. Also, the absorbed wavelength region and the transmitted wavelength region are determined by the mutual relationship between the thicknesses of the oxide layers, and color to be emitted. Generally, if film thickness=(target center wavelength .lambda.c).times.(2n-1)/4 (n=integer number), emission of a target color will take place due to interference. Although not shown in FIG. 2, TFTs for changing the orientation of a liquid crystal are formed on the glass substrate in addition to the multilayered interference film 5.
In a multilayered interference film, reflectance R prescribing requisite brightness is proportional to the square of (n.sub.1 -n.sub.2)/(n.sub.1 +n.sub.2), and n.sub.1 and n.sub.2 are the refractive indexes of the respective films which constitute a multilayered interference film. If the difference .DELTA.n between the refractive indexes of two films of a multilayered interference film is large, sufficient reflectance R will be obtainable to provide bright images. As the value of n.sub.1 -n.sub.2 =.DELTA.n is made larger, brightness increases.
However, color purity is degraded as the difference An between the refractive indexes of two films is enlarged. That is, in a multilayered interference film where the difference An between the refractive indexes of two films is large, the measured wavelength-reflectance dependency has an extremely broad peak of reflectance with respect to the center wavelength. Generally speaking, such a broad wavelength-reflectance characteristic is not sufficient from the point of color purity. Therefore, in the background art a .DELTA.n&lt;1 has been employed. For example, the combination of a titan oxide (TiO.sub.2) and a silicon oxide (SiO.sub.2), shown in FIG. 2, has .DELTA.n=0.9. In such a combination, .DELTA.n becomes large and therefore the reflectance R is small, so a sufficient white level cannot be ensured. FIG. 3 shows the wavelength-reflectance dependency of the multilayered interference film shown in FIG. 2. In each of the red, green, and blue regions, the reflectance is about 60% at its maximum and sufficient reflectance is unobtainable. Therefore, the white level (heavy line) inevitably is too small to be of practical use. Thus, even in the color emission of the multilayered interference film type, as with the color emission of the color filter, the equilibrium between color purity and brightness (white level) is an essential problem. Also, in the multilayered interference film type there is the antinomy that a larger An makes both the reflectance of each of the multilayered interference films and the brightness (white level) larger, but, on the other hand, degrades color purity.
To increase the white level, a five level form of the aforementioned multilayered interference film has been considered. Since the white level depends on the total amount of light reflected by each layer, the absolute amount of light to be reflected is increased by increasing the number of layers. The wavelength-reflectance dependency of a multilayered interference film consisting of five layers is shown in FIG. 4. The five-layer film is formed by simply adding` a layer of SiO.sub.2 and a layer of TiO.sub.2 to the three-layer film shown in FIG. 2. The white level is slightly improved, compared to the three level film, but there is no essential improvement. Therefore, the limitation on brightness (white level) exits in the solution method of forming a multilayered interference of each color in parallel on the glass substrate 1 and increasing the number of layers of the respective multilayered interference films.